Document error with GetHeaders

• Please check this box if this contribution uses AI-generated content as outlined in the GitLab DCO & CLA.

What does this MR do?

The following exception might get printed when DAST_API_REQUEST_HEADERS variable does not contain a ::

08:45:43.544 [DBG] <Peach.Web.Runner.Services.Readers.OpenApiReader> Found OpenApi document version OpenApi3
08:45:43.616 [ERR] <Peach.Web.Core.Services.WebRunnerMachine> Unexpected exception in WebRunnerMachine::Run()
System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at Peach.Web.Runner.Services.RunnerOptions.GetHeaders() in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Runner/Services/[RunnerOptions.cs:line 362
   at Peach.Web.Runner.Services.RunnerService.Start(Job job, IRunnerOptions options) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Runner/Services/RunnerService.cs:line 67
   at Peach.Web.Core.Services.WebRunnerMachine.Run(IRunnerOptions runnerOptions, CancellationToken token) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Core/Services/WebRunnerMachine.cs:line 321
08:45:43.634 [WRN] <Peach.Web.Core.Services.WebRunnerMachine> * Session failed: An unexpected exception occurred: Index was outside the bounds of the array.
08:45:43.677 [INF] <Peach.Web.Core.Services.WebRunnerMachine> Finished testing. Performed a total of 0 requests.

Related issues

Related customer ticket: https://gitlab.zendesk.com/agent/tickets/536464

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.